Yes my point want that such a meeting would be similar, but that such a meeting between alien ecosystems could have a similar effect. To the larger point about the OP, my issue is the idea that any of this is unlikely, when there is a sufficient lack of information to base any sort of probabilistic assumption. They’re all simply incorrect because there is insufficient data. For all we know an alien organism may be specifically evolved to devour multicellular life, or pass into other organisms without detection. There is no reason to think the Ridley Scott scenario is any more or less likely than a harmless scenario. We just don’t know because our frame of reference is only life on Earth.